Heat spreader for emissive display device

ABSTRACT

A heat spreader for an emissive display device, such as a plasma display panel or a light emitting diode, comprising at least one sheet of compressed particles of exfoliated graphite having a surface area greater than the surface area of that part of a discharge cell facing the back surface of the device.

RELATED APPLICATIONS

This application is a continuation-in-part of copending and commonlyassigned U.S. patent application Ser. No. 10/685,103, filed in the namesof Norley, Smalc, Capp and Clovesko on Oct. 14, 2003, entitled “HeatSpreader for Plasma Display Panel,” and copending and commonly assignedU.S. patent application Ser. No. 10/844,537, filed in the names ofClovesko, Norley, Smalc and Capp on May 12, 2004, entitled “HeatSpreader for Emissive Display Device,” the disclosures of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a heat spreader useful for an emissivedisplay device, such as a plasma display panel (PDP) or a light emittingdiode (LED), and the unique thermal issues occasioned by these devices.

BACKGROUND OF THE ART

A plasma display panel is a display apparatus which contains a pluralityof discharge cells, and is constructed to display an image by applying avoltage across electrodes discharge cells thereby causing the desireddischarge cell to emit light. A panel unit, which is the main part ofthe plasma display panel, is fabricated by bonding two glass base platestogether in such a manner as to sandwich a plurality of discharge cellsbetween them.

In a plasma display panel, each of the discharge cells which are causedto emit light for image formation generate heat and each thusconstitutes a source of heat, which causes the temperature of the plasmadisplay panel as a whole to rise. The heat generated in the dischargecells is transferred to the glass forming the base plates, but heatconduction in directions parallel to the panel face is difficult becauseof the properties of the glass base plate material.

In addition, the temperature of a discharge cell which has beenactivated for light emission rises markedly, while the temperature of adischarge cell which has not been activated does not rise as much.Because of this, the panel face temperature of the plasma display panelrises locally in the areas where an image is being generated. Moreover,a discharge cell activated in the white or lighter color spectragenerate more heat than those activated in the black or darker colorspectra. Thus, the temperature of the panel face differs locallydepending on the colors generated in creating the image. These localizedtemperature differentials can accelerate thermal deterioration ofaffected discharge cells, unless measures are taken to ameliorate thedifferences. Additionally, when the nature of the image on the displaychanges, the location for localized heat generation changes with theimage.

Further, since the temperature difference between activated andnonactivated discharge cells can be high, and the temperature differencebetween discharge cells generating white light and those generatingdarker colors also can be high, a stress is applied to the panel unit,causing the conventional plasma display panel to be prone to cracks andbreakage.

When the voltage to be applied to the electrodes of discharge cells isincreased, the brightness of the discharge cells increases but theamount of heat generation in such cells also increases. Thus, thosecells having large voltages for activation become more susceptible tothermal deterioration and tend to exacerbate the breakage problem of thepanel unit of the plasma display panel. LEDs present similar issues withrespect to heat generation as do PDPs.

The use of so-called “high orientation graphite film” as thermalinterface materials for plasma display panels to fill the space betweenthe back of the panel and a heat sinking unit and to even out localtemperature differences is suggested by Morita, Ichiyanagi, Ikeda,Nishiki, Inoue, Komyoji and Kawashima in U.S. Pat. No. 5,831,374,however, no mention of the use or distinct advantages of flexiblegraphite sheets is made. In addition, U.S. Pat. No. 6,482,520 to Tzengdiscloses the use of sheets of compressed particles of exfoliatedgraphite as heat spreaders (referred to in the patent as thermalinterfaces) for a heat source such as an electronic component. Indeed,such materials are commercially available from Advanced EnergyTechnology Inc. of Lakewood, Ohio as its eGraf® SpreaderShield class ofmaterials.

Graphites are made up of layer planes of hexagonal arrays or networks ofcarbon atoms. These layer planes of hexagonally arranged carbon atomsare substantially flat and are oriented or ordered so as to besubstantially parallel and equidistant to one another. The substantiallyflat, parallel equidistant sheets or layers of carbon atoms, usuallyreferred to as graphene layers or basal planes, are linked or bondedtogether and groups thereof are arranged in crystallites. Highly orderedgraphites consist of crystallites of considerable size, the crystallitesbeing highly aligned or oriented with respect to each other and havingwell ordered carbon layers. In other words, highly ordered graphiteshave a high degree of preferred crystallite orientation. It should benoted that graphites possess anisotropic structures and thus exhibit orpossess many properties that are highly directional such as thermal andelectrical conductivity.

Briefly, graphites may be characterized as laminated structures ofcarbon, that is, structures consisting of superposed layers or laminaeof carbon atoms joined together by weak van der Waals forces. Inconsidering the graphite structure, two axes or directions are usuallynoted, to wit, the “c” axis or direction and the “a” axes or directions.For simplicity, the “c” axis or direction may be considered as thedirection perpendicular to the carbon layers. The “a” axes or directionsmay be considered as the directions parallel to the carbon layers or thedirections perpendicular to the “c” direction. The graphites suitablefor manufacturing flexible graphite sheets possess a very high degree oforientation.

As noted above, the bonding forces holding the parallel layers of carbonatoms together are only weak van der Waals forces. Natural graphites canbe treated so that the spacing between the superposed carbon layers orlaminae can be appreciably opened up so as to provide a marked expansionin the direction perpendicular to the layers, that is, in the “c”direction, and thus form an expanded or intumesced graphite structure inwhich the laminar character of the carbon layers is substantiallyretained.

Graphite flake which has been greatly expanded and more particularlyexpanded so as to have a final thickness or “c” direction dimensionwhich is as much as about 80 or more times the original “c” directiondimension can be formed without the use of a binder into cohesive orintegrated sheets of expanded graphite, e.g. webs, papers, strips,tapes, foils, mats or the like (typically referred to as “flexiblegraphite”). The formation of graphite particles which have been expandedto have a final thickness or “c” dimension which is as much as about 80times or more the original “c” direction dimension into integratedflexible sheets by compression, without the use of any binding material,is believed to be possible due to the mechanical interlocking, orcohesion, which is achieved between the voluminously expanded graphiteparticles.

In addition to flexibility, the sheet material, as noted above, has alsobeen found to possess a high degree of anisotropy with respect tothermal conductivity due to orientation of the expanded graphiteparticles and graphite layers substantially parallel to the opposedfaces of the sheet resulting from high compression, making it especiallyuseful in heat spreading applications. Sheet material thus produced hasexcellent flexibility, good strength and a high degree of orientation.

Briefly, the process of producing flexible, binderless anisotropicgraphite sheet material, e.g. web, paper, strip, tape, foil, mat, or thelike, comprises compressing or compacting under a predetermined load andin the absence of a binder, expanded graphite particles which have a “c”direction dimension which is as much as about 80 or more times that ofthe original particles so as to form a substantially flat, flexible,integrated graphite sheet. The expanded graphite particles thatgenerally are worm-like or vermiform in appearance, once compressed,will maintain the compression set and alignment with the opposed majorsurfaces of the sheet. The density and thickness of the sheet materialcan be varied by controlling the degree of compression. The density ofthe sheet material can be within the range of from about 0.04 g/cc toabout 2.0 g/cc.

The flexible graphite sheet material exhibits an appreciable degree ofanisotropy due to the alignment of graphite particles parallel to themajor opposed, parallel surfaces of the sheet, with the degree ofanisotropy increasing upon compression of the sheet material to increaseorientation. In compressed anisotropic sheet material, the thickness,i.e. the direction perpendicular to the opposed, parallel sheet surfacescomprises the “c” direction and the directions ranging along the lengthand width, i.e. along or parallel to the opposed, major surfacescomprises the “a” directions and the thermal and electrical propertiesof the sheet are very different, by orders of magnitude, for the “c” and“a” directions.

However, there is a concern in the electronics industries in generalthat the use of graphite-based materials can result in graphiteparticles flaking off, with the result that the flakes can mechanically(i.e., in the same manner as dust particles) interfere with equipmentoperation and function and, more significantly, that due to theconductive nature of graphite, graphite flakes can electricallyinterfere with operation of the emissive display device. Although it isbelieved that it has been shown that these concerns are misplaced, theystill survive.

Also, the use of adhesives to attach a graphite heat spreader to anemissive display device can be disadvantageous at times. Morespecifically, in the event rework (i.e., removal and replacement of theheat spreader) is needed, the adhesive bond can be stronger than thestructural integrity of the graphite sheet; in this situation, thegraphite sheet cannot always be cleanly lifted off the panel without theuse of scrapers or other like tools, which can be time consuming andpotentially damaging to the graphite sheet, the panel or both.

Thus, what is desired is a light-weight and cost-effective thermalspreader for emissive display devices, especially one which is isolatedto prevent flaking off of graphite particles and which can beeffectively removed from the device when needed. The desired spreadershould be capable of balancing the temperature differences over the areaof the device contacted by the spreader to thereby reduce thermalstresses to which the panel would otherwise be exposed, and to be ableto function to reduce hot spots even where the locations of hot spotsare not fixed.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a heatspreader for an emissive display device such as a plasma display panelor a light emitting diode.

Another object of the present invention is to provide a heat spreadermaterial which can be used in an emissive display device to amelioratethe temperature differences which occur in the panel during use.

Yet another object of the present invention is to provide a heatspreader material to a heat source such as a plasma display panel sothat the temperature difference between any two locations on the panelis reduced as compared to a panel without the inventive heat spreader.

Another object of the present invention is to provide a heat spreadermaterial which can be applied to a heat source or collection of heatsources such as a plasma display panel or light emitting diode andadhere with good thermal contact between the heat spreader and thedevice.

Still another object of the present invention is to provide a heatspreader material which is isolated to prevent or reduce the possibilityof graphite particles flaking off.

Yet another object of the present invention is to provide a heatspreader which can adhered and removed from a heat source with minimaldamage to the spreader or the source.

A further object of the present invention is to provide a heat spreaderwhich can be produced in sufficient volume and in a cost effectivemanner.

These objects and others which will be apparent to the skilled artisanupon reading the following description, can be achieved by providing aheat spreader for an emissive display device, comprising at least onesheet of compressed particles of exfoliated graphite having a surfacearea greater than the surface area of that part of a discharge cellfacing the back surface of the device. The emissive display device canbe a plasma display panel or a light emitting diode panel. Morepreferably, the at least one sheet of compressed particles of exfoliatedgraphite has a surface area greater than the surface area of that partof a plurality of discharge cells facing the back surface of the device.Advantageously, the heat spreader is a laminate comprising a pluralityof sheets of compressed particles of exfoliated graphite, and has aprotective coating thereon to prevent flaking of graphite particlestherefrom. In a preferred embodiment, the surface of the heat spreaderhas a facing sheet, such as an aluminum or copper sheet thereon, tofurther seal the heats spreader and facilitate rework.

In a preferred embodiment, the heat spreader has an adhesive thereon anda release material positioned such that the adhesive is sandwichedbetween the heat spreader and the release material. The release materialand adhesive are selected to permit a predetermined rate of release ofthe release material without causing undesirable damage to the heatspreader. Indeed, the adhesive and release material provide an averagerelease load of no greater than about 40 grams per centimeter at arelease speed of one meter per second, more preferably no greater thanabout 10 grams per centimeter at a release speed of one meter persecond.

Additionally, the adhesive preferably achieves a minimum lap shearadhesion strength of at least about 125 grams per square centimeter,more preferably an average lap shear adhesion strength of at least about700 grams per square centimeter. The adhesive should result in anincrease in through thickness thermal resistance of the adhesive/heatspreader material of not more than about 35% as compared to the heatspreader material itself. The thickness of the adhesive should be nogreater than about 0.015 millimeters (mm), more preferably no greaterthan about 0.005 mm.

It is to be understood that both the foregoing general description andthe following detailed description provide embodiments of the inventionand are intended to provide an overview or framework of understandingand nature and character of the invention as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention and is incorporated in and constitute a part of thespecification. The drawings illustrate various embodiments of theinvention and together with the description serve to describe theprinciples and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially-broken away top perspective view of an embodimentof the inventive heat spreader.

FIG. 2 is a partially-broken away top perspective view of anotherembodiment of the inventive heat spreader.

FIG. 3 is a side plan view of yet another embodiment of the inventiveheat spreader.

FIG. 4 shows a system for the continuous production of resin-impregnatedflexible graphite sheets.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Graphite is a crystalline form of carbon comprising atoms covalentlybonded in flat layered planes with weaker bonds between the planes. Inobtaining source materials such as the above flexible sheets ofgraphite, particles of graphite, such as natural graphite flake, aretypically treated with an intercalant of, e.g. a solution of sulfuricand nitric acid, where the crystal structure of the graphite reacts toform a compound of graphite and the intercalant. The treated particlesof graphite are hereafter referred to as “particles of intercalatedgraphite.” Upon exposure to high temperature, the intercalant within thegraphite decomposes and volatilizes, causing the particles ofintercalated graphite to expand in dimension as much as about 80 or moretimes its original volume in an accordion-like fashion in the “c”direction, i.e. in the direction perpendicular to the crystalline planesof the graphite. The expanded (otherwise referred to as exfoliated)graphite particles are vermiform in appearance, and are thereforecommonly referred to as worms. The worms may be compressed together intoflexible sheets that, unlike the original graphite flakes, can be formedand cut into various shapes and provided with small transverse openingsby deforming mechanical impact.

Graphite starting materials for the flexible sheets suitable for use inthe present invention include highly graphitic carbonaceous materialscapable of intercalating organic and inorganic acids as well as halogensand then expanding when exposed to heat. These highly graphiticcarbonaceous materials most preferably have a degree of graphitizationof about 1.0. As used in this disclosure, the term “degree ofgraphitization” refers to the value g according to the formula:$g = \frac{3.45 - {d(002)}}{0.095}$where d(002) is the spacing between the graphitic layers of the carbonsin the crystal structure measured in Angstrom units. The spacing dbetween graphite layers is measured by standard X-ray diffractiontechniques. The positions of diffraction peaks corresponding to the(002), (004) and (006) Miller Indices are measured, and standardleast-squares techniques are employed to derive spacing which minimizesthe total error for all of these peaks. Examples of highly graphiticcarbonaceous materials include natural graphites from various sources,as well as other carbonaceous materials such as graphite prepared bychemical vapor deposition, high temperature pyrolysis of polymers, orcrystallization from molten metal solutions, and the like. Naturalgraphite is most preferred.

The graphite starting materials for the flexible sheets used in thepresent invention may contain non-graphite components so long as thecrystal structure of the starting materials maintains the requireddegree of graphitization and they are capable of exfoliation. Generally,any carbon-containing material, the crystal structure of which possessesthe required degree of graphitization and which can be exfoliated, issuitable for use with the present invention. Such graphite preferablyhas an ash content of less than twenty weight percent. More preferably,the graphite employed for the present invention will have a purity of atleast about 94%. In the most preferred embodiment, the graphite employedwill have a purity of at least about 98%.

A common method for manufacturing graphite sheet is described by Shaneet al. in U.S. Pat. No. 3,404,061, the disclosure of which isincorporated herein by reference. In the typical practice of the Shaneet al. method, natural graphite flakes are intercalated by dispersingthe flakes in a solution containing e.g., a mixture of nitric andsulfuric acid, advantageously at a level of about 20 to about 300 partsby weight of intercalant solution per 100 parts by weight of graphiteflakes (pph). The intercalation solution contains oxidizing and otherintercalating agents known in the art. Examples include those containingoxidizing agents and oxidizing mixtures, such as solutions containingnitric acid, potassium chlorate, chromic acid, potassium permanganate,potassium chromate, potassium dichromate, perchloric acid, and the like,or mixtures, such as for example, concentrated nitric acid and chlorate,chromic acid and phosphoric acid, sulfuric acid and nitric acid, ormixtures of a strong organic acid, e.g. trifluoroacetic acid, and astrong oxidizing agent soluble in the organic acid. Alternatively, anelectric potential can be used to bring about oxidation of the graphite.Chemical species that can be introduced into the graphite crystal usingelectrolytic oxidation include sulfuric acid as well as other acids.

In a preferred embodiment, the intercalating agent is a solution of amixture of sulfuric acid, or sulfuric acid and phosphoric acid, and anoxidizing agent, i.e. nitric acid, perchloric acid, chromic acid,potassium permanganate, hydrogen peroxide, iodic or periodic acids, orthe like. Although less preferred, the intercalation solution maycontain metal halides such as ferric chloride, and ferric chloride mixedwith sulfuric acid, or a halide, such as bromine as a solution ofbromine and sulfuric acid or bromine in an organic solvent.

The quantity of intercalation solution may range from about 20 to about350 pph and more typically about 40 to about 160 pph. After the flakesare intercalated, any excess solution is drained from the flakes and theflakes are water-washed.

Alternatively, the quantity of the intercalation solution may be limitedto between about 10 and about 40 pph, which permits the washing step tobe eliminated as taught and described in U.S. Pat. No. 4,895,713, thedisclosure of which is also herein incorporated by reference.

The particles of graphite flake treated with intercalation solution canoptionally be contacted, e.g. by blending, with a reducing organic agentselected from alcohols, sugars, aldehydes and esters which are reactivewith the surface film of oxidizing intercalating solution attemperatures in the range of 25°60 C. and 125°60 C. Suitable specificorganic agents include hexadecanol, octadecanol, 1-octanol, 2-octanol,decylalcohol, 1,10 decanediol, decylaldehyde, 1-propanol, 1,3propanediol, ethyleneglycol, polypropylene glycol, dextrose, fructose,lactose, sucrose, potato starch, ethylene glycol monostearate,diethylene glycol dibenzoate, propylene glycol monostearate, glycerolmonostearate, dimethyl oxylate, diethyl oxylate, methyl formate, ethylformate, ascorbic acid and lignin-derived compounds, such as sodiumlignosulfate. The amount of organic reducing agent is suitably fromabout 0.5 to 4% by weight of the particles of graphite flake.

The use of an expansion aid applied prior to, during or immediatelyafter intercalation can also provide improvements. Among theseimprovements can be reduced exfoliation temperature and increasedexpanded volume (also referred to as “worm volume”). An expansion aid inthis context will advantageously be an organic material sufficientlysoluble in the intercalation solution to achieve an improvement inexpansion. More narrowly, organic materials of this type that containcarbon, hydrogen and oxygen, preferably exclusively, may be employed.Carboxylic acids have been found especially effective. A suitablecarboxylic acid useful as the expansion aid can be selected fromaromatic, aliphatic or cycloaliphatic, straight chain or branched chain,saturated and unsaturated monocarboxylic acids, dicarboxylic acids andpolycarboxylic acids which have at least 1 carbon atom, and preferablyup to about 15 carbon atoms, which is soluble in the intercalationsolution in amounts effective to provide a measurable improvement of oneor more aspects of exfoliation. Suitable organic solvents can beemployed to improve solubility of an organic expansion aid in theintercalation solution.

Representative examples of saturated aliphatic carboxylic acids areacids such as those of the formula H(CH₂)_(n)COOH wherein n is a numberof from 0 to about 5, including formic, acetic, propionic, butyric,pentanoic, hexanoic, and the like. In place of the carboxylic acids, theanhydrides or reactive carboxylic acid derivatives such as alkyl esterscan also be employed. Representative of alkyl esters are methyl formateand ethyl formate. Sulfuric acid, nitric acid and other known aqueousintercalants have the ability to decompose formic acid, ultimately towater and carbon dioxide. Because of this, formic acid and othersensitive expansion aids are advantageously contacted with the graphiteflake prior to immersion of the flake in aqueous intercalant.Representative of dicarboxylic acids are aliphatic dicarboxylic acidshaving 2-12 carbon atoms, in particular oxalic acid, fumaric acid,malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid,1,5-pentanedicarboxylic acid, 1,6-hexanedicarboxylic acid,1,10-decanedicarboxylic acid, cyclohexane-1,4-dicarboxylic acid andaromatic dicarboxylic acids such as phthalic acid or terephthalic acid.Representative of alkyl esters are dimethyl oxylate and diethyl oxylate.Representative of cycloaliphatic acids is cyclohexane carboxylic acidand of aromatic carboxylic acids are benzoic acid, naphthoic acid,anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m- andp-tolyl acids, methoxy and ethoxybenzoic acids, acetoacetamidobenzoicacids and, acetamidobenzoic acids, phenylacetic acid and naphthoicacids. Representative of hydroxy aromatic acids are hydroxybenzoic acid,3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid,4-hydroxy-2-naphthoic acid, 5-hydroxy-1-naphthoic acid,5-hydroxy-2-naphthoic acid, 6-hydroxy-2-naphthoic acid and7-hydroxy-2-naphthoic acid. Prominent among the polycarboxylic acids iscitric acid.

The intercalation solution will be aqueous and will preferably containan amount of expansion aid of from about 1 to 10%, the amount beingeffective to enhance exfoliation. In the embodiment wherein theexpansion aid is contacted with the graphite flake prior to or afterimmersing in the aqueous intercalation solution, the expansion aid canbe admixed with the graphite by suitable means, such as a V-blender,typically in an amount of from about 0.2% to about 10% by weight of thegraphite flake.

After intercalating the graphite flake, and following the blending ofthe intercalated graphite flake with the organic reducing agent, theblend can be exposed to temperatures in the range of 25° to 125°60 C. topromote reaction of the reducing agent and intercalated graphite flake.The heating period is up to about 20 hours, with shorter heatingperiods, e.g., at least about 10 minutes, for higher temperatures in theabove-noted range. Times of one-half hour or less, e.g., on the order of10 to 25 minutes, can be employed at the higher temperatures.

The above described methods for intercalating and exfoliating graphiteflake may beneficially be augmented by a pretreatment of the graphiteflake at graphitization temperatures, i.e. temperatures in the range ofabout 3000°60 C. and above and by the inclusion in the intercalant of alubricious additive.

The pretreatment, or annealing, of the graphite flake results insignificantly increased expansion (i.e., increase in expansion volume ofup to 300% or greater) when the flake is subsequently subjected tointercalation and exfoliation. Indeed, desirably, the increase inexpansion is at least about 50%, as compared to similar processingwithout the annealing step. The temperatures employed for the annealingstep should not be significantly below 3000°60 C., because temperatureseven 100°60 C. lower result in substantially reduced expansion.

The annealing of the present invention is performed for a period of timesufficient to result in a flake having an enhanced degree of expansionupon intercalation and subsequent exfoliation. Typically the timerequired will be 1 hour or more, preferably 1 to 3 hours and will mostadvantageously proceed in an inert environment. For maximum beneficialresults, the annealed graphite flake will also be subjected to otherprocesses known in the art to enhance the degree expansion—namelyintercalation in the presence of an organic reducing agent, anintercalation aid such as an organic acid, and a surfactant washfollowing intercalation. Moreover, for maximum beneficial results, theintercalation step may be repeated.

The annealing step of the instant invention may be performed in aninduction furnace or other such apparatus as is known and appreciated inthe art of graphitization; for the temperatures here employed, which arein the range of 3000°60 C., are at the high end of the range encounteredin graphitization processes.

Because it has been observed that the worms produced using graphitesubjected to pre-intercalation annealing can sometimes “clump” together,which can negatively impact area weight uniformity, an additive thatassists in the formation of “free flowing” worms is highly desirable.The addition of a lubricious additive to the intercalation solutionfacilitates the more uniform distribution of the worms across the bed ofa compression apparatus (such as the bed of a calender stationconventionally used for compressing (or “calendering”) graphite wormsinto flexible graphite sheet. The resulting sheet therefore has higherarea weight uniformity and greater tensile strength, even when thestarting graphite particles are smaller than conventionally used. Thelubricious additive is preferably a long chain hydrocarbon. Otherorganic compounds having long chain hydrocarbon groups, even if otherfunctional groups are present, can also be employed.

More preferably, the lubricious additive is an oil, with a mineral oilbeing most preferred, especially considering the fact that mineral oilsare less prone to rancidity and odors, which can be an importantconsideration for long term storage. It will be noted that certain ofthe expansion aids detailed above also meet the definition of alubricious additive. When these materials are used as the expansion aid,it may not be necessary to include a separate lubricious additive in theintercalant.

The lubricious additive is present in the intercalant in an amount of atleast about 1.4 pph, more preferably at least about 1.8 pph. Althoughthe upper limit of the inclusion of lubricous additive is not ascritical as the lower limit, there does not appear to be any significantadditional advantage to including the lubricious additive at a level ofgreater than about 4 pph.

The thus treated particles of graphite are sometimes referred to as“particles of intercalated graphite.” Upon exposure to high temperature,e.g. temperatures of at least about 160°60 C. and especially about700°60 C. to 1000IC and higher, the particles of intercalated graphiteexpand as much as about 80 to 1000 or more times their original volumein an accordion-like fashion in the c-direction, i.e. in the directionperpendicular to the crystalline planes of the constituent graphiteparticles. The expanded, i.e. exfoliated, graphite particles arevermiform in appearance, and are therefore commonly referred to asworms. The worms may be compression molded together into flexible sheetshaving small transverse openings that, unlike the original graphiteflakes, can be formed and cut into various shapes, as hereinafterdescribed.

Alternatively, the flexible graphite sheets of the present invention mayutilize particles of reground flexible graphite sheets rather thanfreshly expanded worms. The sheets may be newly formed sheet material,recycled sheet material, scrap sheet material, or any other suitablesource.

Also the processes of the present invention may use a blend of virginmaterials and recycled materials, or all recycled materials.

The source material for recycled materials may be sheets or trimmedportions of sheets that have been compression molded as described above,or sheets that have been compressed with, for example, pre-calenderingrolls. Furthermore, the source material may be sheets or trimmedportions of sheets that have been impregnated with resin, but not yetcured, or sheets or trimmed portions of sheets that have beenimpregnated with resin and cured. The source material may also berecycled flexible graphite PEM fuel cell components such as flow fieldplates or electrodes. Each of the various sources of graphite may beused as is or blended with natural graphite flakes.

Once the source material of flexible graphite sheets is available, itcan then be comminuted by known processes or devices, such as a jetmill, air mill, blender, etc. to produce particles. Preferably, amajority of the particles have a diameter such that they will passthrough 20 U.S. mesh; more preferably a major portion (greater thanabout 20%, most preferably greater than about 50%) will not pass through80 U.S. mesh. Most preferably the particles have a particle size of nogreater than about 20 mesh.

The size of the comminuted particles may be chosen so as to balancemachinability and formability of the graphite article with the thermalcharacteristics desired. Thus, smaller particles will result in agraphite article which is easier to machine and/or form, whereas largerparticles will result in a graphite article having higher anisotropy,and, therefore, greater in-plane electrical and thermal conductivity.

Once the source material is comminuted, and any resin is removed ifdesired, it is then re-expanded. The re-expansion may occur by using theintercalation and exfoliation process described above and thosedescribed in U.S. Pat. No. 3,404,061 to Shane et al. and U.S. Pat. No.4,895,713 to Greinke et al.

Typically, after intercalation the particles are exfoliated by heatingthe intercalated particles in a furnace. During this exfoliation step,intercalated natural graphite flakes may be added to the recycledintercalated particles. Preferably, during the re-expansion step theparticles are expanded to have a specific volume in the range of atleast about 100 cc/g and up to about 350 cc/g or greater. Finally, afterthe re-expansion step, the re-expanded particles may be compressed intoflexible sheets, as hereinbefore described.

Flexible graphite sheet and foil are coherent, with good handlingstrength, and are suitably compressed by, e.g. compression molding, to athickness of about 0.025 mm to 3.75 mm and a typical density of about0.1 to 1.5 grams per cubic centimeter (g/cc). Although not alwayspreferred, the flexible graphite sheet can also, at times, beadvantageously treated with resin and the absorbed resin, after curing,enhances the moisture resistance and handling strength, i.e. stiffness,of the flexible graphite sheet as well as “fixing” the morphology of thesheet. When used, a suitable resin content is preferably at least about5% by weight, more preferably about 10 to 35% by weight, and suitably upto about 60% by weight. Resins found especially useful in the practiceof the present invention include acrylic-, epoxy- and phenolic-basedresin systems, or mixtures thereof. Suitable epoxy resin systems includethose based on diglycidyl ether or bisphenol A (DGEBA) and othermultifunctional resin systems; phenolic resins that can be employedinclude resole and novolak phenolics.

With reference to FIG. 4, a system is disclosed for the continuousproduction of resin-impregnated flexible graphite sheet, where graphiteflakes and a liquid intercalating agent are charged into reactor 104.More particularly, a vessel 101 is provided for containing a liquidintercalating agent. Vessel 101, suitably made of stainless steel, canbe continually replenished with liquid intercalant by way of conduit106. Vessel 102 contains graphite flakes that, together withintercalating agents from vessel 101, are introduced into reactor 104.The respective rates of input into reactor 104 of intercalating agentand graphite flake are controlled, such as by valves 108, 107. Graphiteflake in vessel 102 can be continually replenished by way of conduit109. Additives, such as intercalation enhancers, e.g., trace acids, andorganic chemicals may be added by way of dispenser 110 that is meteredat its output by valve 111.

The resulting intercalated graphite particles are soggy and acid coatedand are conducted (such as via conduit 112) to a wash tank 114 where theparticles are washed, advantageously with water which enters and exitswash tank 114 at 116, 118. The washed intercalated graphite flakes arethen passed to drying chamber 122 such as through conduit 120. Additivessuch as buffers, antioxidants, pollution reducing chemicals can be addedfrom vessel 119 to the flow of intercalated graphite flake for thepurpose of modifying the surface chemistry of the exfoliate duringexpansion and use and modifying the gaseous emissions which cause theexpansion.

The intercalated graphite flake is dried in dryer 122, preferably attemperatures of about 75°60 C. to about 150°60 C., generally avoidingany intumescence or expansion of the intercalated graphite flakes. Afterdrying, the intercalated graphite flakes are fed as a stream into flame200, by, for instance, being continually fed to collecting vessel 124 byway of conduit 126 and then fed as a stream into flame 200 in expansionvessel 128 as indicated at 2. Additives such as ceramic fiber particlesformed of macerated quartz glass fibers, carbon and graphite fibers,zirconia, boron nitride, silicon carbide and magnesia fibers, naturallyoccurring mineral fibers such as calcium metasilicate fibers, calciumaluminum silicate fibers, aluminum oxide fibers and the like can beadded from vessel 129 to the stream of intercalated graphite particlespropelled by entrainment in a non-reactive gas introduced at 127.

The intercalated graphite particles 2, upon passage through flame 200 inexpansion chamber 201, expand more than 80 times in the “c” directionand assume a “worm-like” expanded form 5; the additives introduced from129 and blended with the stream of intercalated graphite particles areessentially unaffected by passage through the flame 200. The expandedgraphite particles 5 may pass through a gravity separator 130, in whichheavy ash natural mineral particles are separated from the expandedgraphite particles, and then into a wide topped hopper 132. Separator130 can be by-passed when not needed.

The expanded, i.e., exfoliated graphite particles 5 fall freely inhopper 132 together with any additives, and are randomly dispersed andpassed into compression station 136, such as through trough 134.Compression station 136 comprises opposed, converging, moving porousbelts 157, 158 spaced apart to receive the exfoliated, expanded graphiteparticles 5. Due to the decreasing space between opposed moving belts157, 158, the exfoliated expanded graphite particles are compressed intoa mat of flexible graphite, indicated at 148 having thickness of, e.g.,from about 25.4 to 0.075 mm, especially from about 25.4 to 2.5 mm, and adensity of from about 0.08 to 2.0 g/cm³. Gas scrubber 149 may be used toremove and clean gases emanating from the expansion chamber 201 andhopper 132.

The mat 148 is passed through vessel 150 and is impregnated with liquidresin from spray nozzles 138, the resin advantageously being “pulledthrough the mat” by means of vacuum chamber 139 and the resin isthereafter preferably dried in dryer 160 reducing the tack of the resinand the resin impregnated mat 143 is thereafter densified into rollpressed flexible graphite sheet 147 in calender mill 170. Gases andfumes from vessel 150 and dryer 160 are preferably collected and cleanedin scrubber 165.

After densification, the resin in flexible graphite sheet 147 is atleast partially cured in curing oven 180. Alternatively, partial curecan be effected prior to densification, although post-densification cureis preferred.

In one embodiment of the invention, however, the flexible graphite sheetis not resin-impregnated, in which case vessel, 150, dryer 160 andcuring oven 180 can be eliminated.

Plasma display panels are now being produced at sizes of 1 meter andabove (measured from corner to corner). Thus, heat spreaders used tocool and ameliorate the effects of hot spots on such panels are alsorequired to be relatively large, on the order of about 270millimeters×about 500 millimeters, or as large as about 800millimeters×500 millimeters, or even larger. In a plasma display panel,as discussed above, thousands of cells, each containing a plasma gas,are present. When a voltage is applied to each cell, the plasma gas thenreacts with phosphors in each cell to produce colored light. Sincesignificant power is required to ionize the gas to produce the plasma,the plasma display can become very hot. Moreover, depending on the colorin a particular region of the panel, hot spots can be created on thescreen which can result in premature breakdown of the phosphors whichcan shorten display life as well as cause thermal stresses on the panelitself. Therefore, a heat spreader is needed to reduce the effect ofthese hot spots.

Sheets of compressed particles of exfoliated graphite, especiallylaminates of sheets of compressed particles of exfoliated graphite, havebeen found particularly useful as heat spreaders for plasma displaypanels. More particularly, one or more sheets of compressed particles ofexfoliated graphite, referred to herein as sheets of flexible graphite,are placed in thermal contact with the back of a plasma display panel,such that the flexible graphite sheet overlays a plurality of heatsources (i.e., discharge cells) in the panel. In other words, thesurface area of the flexible graphite sheet is larger than the surfacearea of a discharge cell at the back of the plasma display panel;indeed, the surface area of the flexible graphite sheet is larger thanthe surface area of a plurality of discharge cells at the back of theplasma display panel. Thus, because of the nature of the flexiblegraphite material from which the inventive heat spreader is formed, itwill spread the heat from hot spots which may arise in differentlocations on the plasma display panel, as the image displayed by thepanel changes.

Because of the nature of flexible graphite sheet materials, in that theyare more conformable than other materials, even other forms of graphite,the contact resistance between the heat spreader and the plasma displaypanel is reduced and better thermal contact can be achieved than whenusing prior art heat spreaders applied with equivalent applicationpressures.

The flexible graphite sheet heat spreader of the present invention actsto reduce the heat difference (i.e., ΔT) between locations on the plasmadisplay panel. In other words, the temperature difference between a hotspot on the panel, such as a location where a white image is created,and an adjacent location where a darker image is created, is reduced bythe use of the inventive flexible graphite heat spreaders, as comparedto the ΔT had the flexible graphite sheet not been present. Therefore,thermal stresses to which the plasma display panel would otherwise havebeen exposed are reduced, extending panel life and effectiveness.Moreover, since hot spots (i.e. thermal spikes) are reduced, the entireunit can be run at a higher temperature, with resulting imageimprovement.

In practice, it may be advantageous for the graphite heat spreaders tobe produced with a layer of adhesive thereon to adhere the heat spreaderto the plasma display panel, especially during the plasma display panelassembly process. A release liner must then be used to overlay theadhesive, with the adhesive sandwiched between the release liner and thegraphite sheet, to permit storage and shipping of the graphite heatspreader prior to adhesion to the plasma display panel.

The use of an adhesive coated graphite sheet (or laminate of sheets)with a release liner has certain requirements which should be met if itis to be practical in a high volume plasma display panel manufacturingprocess. More particularly, the release liner must be capable of beingremoved from the sheet at high speed without causing delamination of thegraphite. Delamination occurs when the release liner in effect pulls theadhesive and some of the graphite off the sheet as it is being removed,resulting in a loss of graphite, impairment of the graphite sheetitself, and diminution of adhesive needed to adhere the graphite sheetto the plasma display panel, as well as an unsightly and unfortunateappearance.

With that however, though the adhesive and release liner should beselected to permit release of the release liner from theadhesive/graphite sheet without delamination of the graphite, theadhesive must still be strong enough to maintain the graphite sheet inposition on the plasma display panel while the panel assumes any of avariety of orientations and to ensure good thermal contact between theheat spreader(s) and the panel.

In addition, significant diminution of the thermal performance of theheat spreader must not be caused by the adhesive. In other words, anadhesive applied in a layer that is of substantial thickness caninterfere with the thermal performance of the heat spreader, since theadhesive would interfere with the conduction of heat from the plasmadisplay panel to the heat spreader.

Thus, the adhesive and release liner combination must achieve a balancesuch that they provide a release load no greater than about 40 g/cm,more preferably about 20 g/cm and most preferably about 10 g/cm, at arelease speed of about 1 m/s, as measured, for instance, on aChemInstruments HSR-1000 high speed release tester. For example, if itis desired to remove the release liner at a speed of about 1 m/s inorder to match the high volume manufacturing requirements of the plasmadisplay panel, the average release load of the release liner should beno greater than about 40 g/cm, more advantageously, about 20 g/cm, andmost advantageously about 10 g/cm, in order to permit removal of therelease liner without causing graphite delamination at that releasespeed. To achieve this, the adhesive should preferably be no greaterthan about 0.015 mm, most preferably no greater than about 0.005 mm, inthickness.

Another factor to be balanced is the adhesion strength of the adhesivewhich as noted above, should be sufficient to maintain the heat spreaderin position on the plasma display panel during the plasma display panelmanufacturing process and to ensure good thermal contact between theheat spreader and the plasma display panel. In order to achieve therequired adhesion, the adhesive should have a minimum lap shear adhesionstrength of at least about 125 g/cm², more preferably an average lapshear adhesion strength of at least about 700 g/cm², as measured, forinstance, on a ChemInstruments TT-1000 tensile tester.

With all that, as noted above, the adhesive should not substantiallyinterfere with the thermal performance of the heat spreader. By this ismeant, the presence of the adhesive should not result in an increase inthe through-thickness thermal resistance of the heat spreader of morethan about 100% as compared to the heat spreader material itself,without adhesive. Indeed, in the more preferred embodiment, the adhesivewill not lead to an increase in the thermal resistance of more thanabout 35% as compared to the heat spreader material without adhesive.Thus, the adhesive must meet the release load requirements and averagelap shear adhesion strength requirement while being thin enough to avoidan undesirably high increase in thermal resistance. In order to do so,the adhesive should be no thicker than about 0.015 mm, more preferablyno thicker than about 0.005 mm.

In order to achieve the balancing described above needed for theproduction of a heat spreader useful for being applied to a plasmadisplay panel in a high volume manufacturing process, where the heatspreader is a sheet or laminate of sheets of compressed particles ofexfoliated graphite having a thickness no greater than about 2.0 mm anda density between about 1.6 and about 1.9 grams per cubic centimeter, acommercially available pressure sensitive acrylic adhesive in thedesired thickness combined with a release liner made of silicone-coatedKraft paper such as an L2 or L4 release liner commercially availablefrom Sil Tech, a division of Technicote Inc., can achieve the desiredresults. Thus, a heat spreader composite is provided which comprises aheat spreader material such as a sheet or laminate of sheets ofcompressed particles of exfoliated graphite, having an adhesive thereonin a thickness such that the thermal performance of the heat spreadermaterial is not substantially compromised, with a release layerpositioned such that the adhesive is sandwiched between the heatspreader material and the release material. In operation then, therelease material can be removed from the heat spreader/adhesivecombination and the heat spreader material/adhesive combination thenapplied to a plasma display panel such that the adhesive adheres theheat spreader material to the plasma display panel. Furthermore, when aplurality of plasma display panels is being produced, the at least oneof the heat spreader/adhesive combinations is applied to each of theplurality of plasma display panels.

When a flexible graphite laminate is employed as the inventive heatspreader, other laminate layers can also be included, to improve themechanical or thermal properties of the laminate. For instance, alaminate layer of a thermally conductive metal like aluminum or coppercan be interposed between layers of flexible graphite in order toincrease the thermal spreading characteristics of the laminate withoutsacrificing the low contact resistance exhibited by graphite; othermaterials, such as polymers, can also be employed to reinforce orotherwise improve the strength of the laminate. In addition, thegraphite material, whether a single sheet or a laminate, can be providedwith a backing layer of, for instance, a thin plastic sheet or, in thealternative, a thin coating of dried resin, to improve handling of thematerial and/or reduce damage to the sheet during shipment orapplication to the panel, without compromising the thermal spreadingcapabilities of the inventive heat spreader. A layer of an insulatingmaterial can also be employed.

In addition, the surface of the heat spreader destined to abut theplasma display panel can have a facing of a material to improve thermalperformance and/or the ability to rework the inventive heat spreader.Most preferred is a metal like aluminum or copper, with aluminum mostpreferred. Although there may be some thermal sacrifice in terms ofgreater contact resistance (because the conformable graphite surface isnot in contact with the panel surface when such a facing is used), thatcan be made up for by the thermal isotropy of the metallic facing. Moreto the point, however, since it is the facing which would be adhered tothe panel surface, removal of the inventive heat spreader for rework orotherwise is facilitated, since the structure of a metal facing would bestronger than the adhesive bond, allowing for quick and non-damagingremoval of the heat spreader from the plasma display panel surface.

As illustrated in FIG. 1, once formed, the flexible graphite sheet orlaminate intended for use as the inventive heat spreader, denoted 10,can be cut into the desired shape, in most cases rectangular. Heatspreader 10 has two major surfaces 12 and 14, as well as at least oneedge surface 16, and generally four edge surfaces 16 a, 16 b, 16 c, 16 dif spreader 10 is rectangular (as would be apparent, when heat spreader10 is cut into other than a square shape, such as a round shape or amore complex shape, it will have different numbers of edge surfaces 16).

Referring now to FIGS. 1-3, heat spreader 10 can advantageously alsocomprise a protective coating 20, to forestall the possibility ofgraphite particles flaking from, or otherwise being separated from, theflexible graphite sheet or laminate which makes up heat spreader 10.Protective coating 20 also advantageously effectively isolates heatspreader 10, to avoid electrical interference engendered by theinclusion of an electrically conductive material (graphite) in anelectronic device. Protective coating 20 can comprise any suitablematerial sufficient to prevent the flaking of the graphite materialand/or to electrically isolate the graphite, such as a thermoplasticmaterial like polyethylene, a polyester or a polyimide, a wax and/or avarnish material. Indeed, when grounding is desired, as opposed toelectrical isolation, protective coating 20 can comprise a metal such asaluminum.

Advantageously, to achieve the desired flake-resistance and/orelectrical isolation, protective coating 20 should preferably be atleast about 0.001 mm thick. Although there is no true maximum thicknessfor protective coating 20, protective coating 20 should be no more thanabout 0.025 mm thick, preferably no more than about 0.005 mm thick tofunction effectively.

When heat spreader 10 is applied to the plasma display panel, majorsurface 12 of heat spreader 10 is that surface which is in operativecontact with the panel. As such, in many applications, the contactbetween major surface 12 and the panel will function to “seal” majorsurface 12 against graphite flaking, thus eliminating the need to coatmajor surface 12 with protective coating 20. Likewise, provided majorsurface 14 is electrically isolated from the rest of the electricaldevice in which heat spreader 10 is located, electrically isolatingmajor surface 12 is not necessary. However, for handling or otherconsiderations, in some embodiments protective coating 20 can be coatedon both major surfaces 12 and 14 of heat spreader 10 be interposedbetween the graphite sheet and any adhesive used on major surface 12 foradhering heat spreader 10 to the plasma display panel (not shown).

Heat spreader 10 can be provided with protective coating 20 by severaldifferent processes. For instance, once the flexible graphite sheet orlaminate is cut to size and shape to form heat spreader 10, the materialfrom which protective coating 20 is formed can be coated on theindividual heat spreader 10 so as to flow completely about major surface14 and edge surface 16, etc. and extend beyond edge surface 16, etc. toform a protective flaking boundary about heat spreader 10, asillustrated in FIG. 1. To that end, protective coating 20 can be appliedby various coating methods familiar to the skilled artisan, such asspray coating, roller coating and hot laminating press.

In an alternative embodiment, illustrated in FIG. 2, protective coating20 can be applied to heat spreader 10 so as to cover one or more edgesurfaces 16 a, 16 b, 16 c, 16 d (depending, for instance, on which areexposed and thus potentially subject to flaking and/or causingelectrical interference). Protective coating 20 can be applied bymechanical mapping and lamination to accomplish this.

In yet another embodiment of the present invention, and as shown in FIG.3, protective coating 20 is applied to heat spreader 10 so as to coatmajor surface 14 only. One particularly advantageous way ofmanufacturing this embodiment of heat spreader 10 is to coat a flexiblegraphite sheet or laminate with protective coating 20, such as by rollercoating, laminating with adhesive, or hot press laminating, and thencutting the flexible graphite sheet or laminate into the desired shapeof heat spreader 10. In this way, manufacturing efficiency is maximizedand waste of protective coating 20 minimized in the manufacturingprocess.

Generally, the coating process adheres protective coating 20 to heatspreader 10 with sufficient strength for most applications. However, ifdesired, or for relatively non-adhesive protective coatings 20, such asMylar® polyester materials and Kapton polyimide materials (bothcommercially available from E.I. du Pont de Nemours and Company ofWilmington, Del.), a layer of adhesive 30 may be applied between heatspreader 10 and protective coating 20, as illustrated by FIG. 3.Suitable adhesives are those which can facilitate the adhesion ofprotective coating 20 to heat spreader 10, such as acrylic or latexadhesives. Layer of adhesive 30 can be coated on either or both of heatspreader 10 and protective coating 20. Advantageously, layer of adhesive30 is as thin as possible while still maintaining adhesion betweenprotective coating 20 and heat spreader 10. Preferably, layer ofadhesive 30 is no more than about 0.015 mm in thickness.

In addition, in another embodiment, heat spreader 10 can comprise afacing layer 40 which is interposed between surface 12 of heat spreader10 and the surface of the plasma display panel. As discussed above,facing layer 40 is preferably a metal, such as aluminum, and can beadhered to surface 12 by use of a layer of adhesive 50 applied betweensurface 12 of heat spreader 10 and facing layer 40, as illustrated byFIG. 1. Suitable adhesives are acrylic or latex adhesives, and can becoated on either or both of heat spreader surface 12 and facing layer40. Of course, the adhesive 50 is applied as thin as possible whilestill maintaining adhesion between facing layer 40 and surface 12,preferably no more than about 0.015 mm in thickness.

In addition, as shown in FIG. 1, facing layer 40 can cooperate withprotective coating 20 to seal graphite heat spreader 10 between facinglayer 40 and protective coating 20. More specifically, if facing layer40 extends beyond edges 16, etc. of spreader 10, protective coating canbe coated about heat spreader 10 and to facing layer 40. Alternatively,a material such as aluminum tape can be employed to seal edges 16 etc.between facing layer 40 and protective coating 20.

Although this application is written in terms of the application of heatspreaders to plasma display panels, it will be recognized that theinventive method and heat spreader are equally applicable to otheremissive display device heat sources, or heat source collections(equivalent in relevant function to the collection of individualdischarge cells making up the plasma display panel) such as lightemitting diodes.

All cited patents and publications referred to in this application areincorporated by reference.

The invention thus being described, it will obvious that it may bevaried in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the present invention and allsuch modifications as would be obvious to one skilled in the art areintended to be included in the scope of the following claims.

1. A heat spreader for an emissive display device, comprising at leastone sheet of compressed particles of exfoliated graphite having asurface area greater than the surface area of that part of a dischargecell facing the back surface of the emissive display device.
 2. The heatspreader of claim 1 wherein the emissive display device comprises aplasma display panel.
 3. The heat spreader of claim 2 wherein the atleast one sheet of compressed particles of exfoliated graphite has asurface area greater than the surface area of that part of a pluralityof discharge cells facing the back surface of the plasma display panel4. The heat spreader of claim 1 wherein the heat spreader comprises alaminate comprising a plurality of sheets of compressed particles ofexfoliated graphite.
 5. The heat spreader of claim 4 wherein thelaminate comprises layers of a non-graphitic material.
 6. The heatspreader of claim 1 wherein the heat spreader comprises an adhesivethereon and a release material positioned such that the adhesive issandwiched between the heat spreader and the release material.
 7. Theheat spreader of claim 6 wherein the release material and adhesive areselected to permit a predetermined rate of release of the releasematerial without causing undesirable damage to the heat spreader.
 8. Theheat spreader of claim 7 wherein the adhesive and release materialprovide an average release load of no greater than about 40 grams percentimeter at a release speed of one meter per second.
 9. The heatspreader of claim 8 wherein the adhesive achieves a minimum lap shearadhesion strength of at least about 125 grams per square centimeter. 10.The heat spreader of claim 9 wherein the adhesive achieves an averagelap shear adhesion strength of at least about 700 grams per squarecentimeter.
 11. The heat spreader of claim 10 wherein the thickness ofthe adhesive is no greater than about 0.015 mm.
 12. The heat spreader ofclaim 1 wherein at least one of the major surfaces is coated with aprotective coating sufficient to inhibit flaking of particles ofgraphite.
 13. The heat spreader of claim 12 wherein the protectivecoating comprises a metal or a thermoplastic material.
 14. The heatspreader of claim 13 wherein the thermoplastic material comprisespolyethylene, a polyester or a polyimide.
 15. The heat spreader of claim14 wherein the protective coating is no more than about 0.025 mm inthickness.
 16. The heat spreader of claim 15 wherein the protectivecoating is effective to electrically isolate the coated major surface ofthe at least one sheet of compressed particles of exfoliated graphite.17. The heat spreader of claim 12 wherein the at least one sheet ofcompressed particles of exfoliated graphite has edge surfaces, and atleast one edge surface is coated with a protective coating sufficient toinhibit flaking of the particles of graphite.
 18. The heat spreader ofclaim 12 which further comprises a layer of adhesive interposed betweenthe protective coating and the at least one sheet of compressedparticles of exfoliated graphite.
 19. The heat spreader of claim 1 whichfurther comprises a facing layer adhered to a surface thereof for beinginterposed between the sheet of compressed particles of exfoliatedgraphite and the emissive display device.
 20. The heat spreader of claim19 wherein the facing layer comprises a metal.